The present invention relates to a satellite communications apparatus and method, and particularly but not exclusively to apparatus connectable or connected to a communications terminal to enable communication with a geostationary or quasi-geostationary satellite.
The term xe2x80x98quasi-geostationaryxe2x80x99 includes satellites which, individually or collectively, do not move significantly in elevation or bearing relative to a user during a communications session and which do not require accurate knowledge by the user of their position as a function of time. Thus, the satellites may be geosynchronous with a small orbital inclination relative to the equator, so that the deviation in latitude of the satellites is not significant to the user. Alternatively, the satellites may be in highly elliptical orbits such as the LOOPUS orbit in which the satellites dwell over an area of the earth""s surface for several hours around their apogee. With such orbits, individual satellites may move significantly relative to the user provided that calls are handed off to another satellite so that there is always a satellite available to the user within a range of positions which can be covered by the user antenna without adjustment during a communications session.
In satellite communications systems which use geostationary satellites, user terminals commonly communicate with the satellites by means of directional antennas, in order to provide a satisfactory gain in the communications link to and from the satellite. The directional antenna must be steered towards the geostationary satellite.
One example of such a system is the Inmarsat-B(trademark) system, designed primarily for use with ship-based terminals. The antenna assemblies for these terminals are large, typically comprising a 0.9 m diameter parabolic antenna with stabilization and automatic satellite tracking mechanisms.
Another example of such a system is the Inmarsat-M(trademark) system, which shares many of the design features of Inmarsat-B(trademark), but is able to support more compact user terminals, including portable terminals the size of a briefcase.
The advent of geostationary satellites, such as the Inmarsat-3(trademark) satellites, with multiple spot beams per satellite and higher power and sensitivity has further reduced the minimum gain requirements of user terminals for use with such satellites. It is therefore possible to provide high-bandwidth communication services to a user terminal the size of a laptop computer. However, the mechanism required for satellite tracking cannot be miniaturized to the same extent. Therefore, antennas for portable satellite terminals are steered manually towards the satellite.
The document EP 0 570 325 describes a portable satellite communications terminal in which the antenna is flat and housed in the lid of a briefcase, together with a radio-frequency (RF) transmitter/receiver, which is connected to a laptop computer. The briefcase lid can be retained at different inclinations so as to point the antenna towards the satellite; azimuthal orientation is achieved by rotating the briefcase. Manual pointing is assisted by inputting the user""s longitude and latitude into the computer, which then displays the correct azimuth and elevation angle for the antenna. However, even if the user knows the azimuth and elevation of the satellite, it is not a simple matter to point the antenna in that direction.
The document U.S. Pat. No. 5,347,286 discloses an alternative approach, in which the pointing of an antenna at a satellite is automated by means of a GPS receiver and two GPS antennas mounted on the communications antenna. This approach requires at least two servo motors and associated gear assemblies to steer the antenna in elevation and azimuth. The whole antenna assembly is intended to fit into a suitcase, while the communications terminal itself must be carried in another case. Hence, the equipment required is inconvenient for personal mobile communications.
According to one aspect of the present invention, there is provided a portable satellite communications antenna with an additional antenna mounted thereon for receiving navigation signals, such as GPS or GLONASS signals. The inclination of the communications antenna can be manually adjusted to point at a geostationary or quasi-geostationary satellite while the navigation antenna is adjusted to point directly upwards.
With the above arrangement, satellite communications and navigation equipment can be conveniently integrated, while allowing both the navigation and communications antennas to be pointed in the optimum direction.
Preferably, the navigation antenna can be stowed within or against the communications antenna assembly for ease of carrying or storage.
According to another aspect of the present invention, there is provided apparatus for satellite communication in which the radio frequency transmitter/receiver is divided into two discrete parts: an intermediate frequency part which converts baseband signals into intermediate frequency signals and vice versa, and an RF conversion part which converts the intermediate frequency signals into RF signals and vice versa. The intermediate frequency part may be integrated with interface circuitry for connection to a general purpose computer, while the RF conversion part may be integrated with a satellite antenna assembly. The intermediate frequency signals are carried between the parts by suitable connection means such as a cable, suitable cable connectors being provided at each of the parts.
The above separation of intermediate and radio frequency parts is advantageous for the following reasons. The placement of the radio frequency part close to the antenna reduces the loss involved in passing RF signals down a cable and the need to use expensive coaxial cable for this connection. The power amplification requirements of the RF stage are also reduced. Furthermore, the separation of the RF stage from the interface stage reduces interference in the RF stage from the internal circuitry of the computer. In addition, the intermediate frequency stage can be miniaturized sufficiently to be contained within a small interface card, such as a PCMCIA card, for use with portable computers.
Moreover, the same intermediate frequency stage may be connected to different RF stages and antennas, which may be required if the same communications terminal is to be used with different systems or in different countries where different frequency bands are used for satellite communications.
According to another aspect of the present invention, there is provided satellite communications apparatus which senses the orientation of a satellite communications antenna, compares this orientation with the correct orientation for satellite communications, and indicates to the user how the orientation should be adjusted to achieve the correct orientation. This arrangement greatly facilitates the setup of a satellite communications antenna, since the user does not have to deal with any absolute measures of direction, but merely adjusts the antenna as indicated until it is pointed correctly. The indication may be performed by a display of a computer connected to the communications apparatus, or by a separate indicator located close to the antenna so that the user need not look at the computer display while adjusting the antenna.